Abstract
Differences in nitrate-N concentrations in ground water in Kansas can be explained by variations in agricultural practices and vadose-zone stratigraphy. In northwestern Kansas, past use of a local stream for tailwater runoff from irrigation and high fertilizer applications for sugar-beet farming resulted in high nitrate-N concentrations (12–60 mg L−1; in both soil and ground water. Nitrogen isotope values from the soil and ground water range from +4 to +8‰, which is typical for a fertilizer source. In parts of south-central Kansas, the use of crop rotation and the presence of both continuous fine-textured layers and a reducing ground-water chemistry resulted in ground-water nitrate-N values of < 3 mg L−1;. The effects of denitrification in the vadose zone and ground water are indicated by enriched δ 15N values of +10 to +15‰. At a site study, irrigated continuous corn was grown on sandy soils with discontinuous fine-textured layers. Here, nitrate-N concentrations were often > 10 mg L−1; in both soil and grounwater. Nitrogen isotope values of +3 to +7‰ indicate a fertilizer source. Crop rotation decreased nitrate-N values in the shallow ground water (9 m). However, deeper ground water showed increasing nitrate-N concentrations as a result of past farming practices.
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